Regular expression for Asp.net text box conrol

I have a text box control and I need to restrict the text box input to no more than 255 characters. I created
a regular expression, however it is not correct. Can someone help me out? Here is my validation expression:
validationexpression="^.{0,255}$">

My actual Asp.net code looks something like, this but I know that the regular expression is not correct.

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